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February 02/2007 National Geographic Image

Flowing Border

Texas catches the sun and Chihuahua lies in shadow along the twisting border line of the Rio Grande. The river cuts its path between thick bands of volcanic rocks, left behind by massive eruptions to the north and south between 27 and 32 million years ago. An outcrop on the Texas side, in Big Bend Ranch State Park, shows distinct bands of light-colored tuff, remnants of ancient ash falls.